This invention pertains to the subject of ophthalmic lenses, and in particular contact lenses containing more than one optical power or focal length.
It is well known that as an individual ages, the eye is less able to accommodate, i.e., bend the natural lens in the eye in order to focus on objects that are relatively near to the observer. This condition is referred to as presbyopia, and presbyopes have in the past relied upon spectacles or other lenses having a number of different regions with different optical powers to which the wearer can shift his vision in order to find the appropriate optical power for the object or objects upon which the observer wishes to focus.
With spectacles this process involves shifting one's field of vision from typically an upper, far power to a different, near power. With contact lenses, however, this approach has been less than satisfactory. The contact lens, working in conjunction with the natural lens, forms an image on the retina of the eye by focusing light incident on each part of the cornea from different field angles onto each part of the retina in order to form the image. This is demonstrated by the fact that as the pupil contracts in response to brighter light, the image on the retina does not shrink, but rather, light coming through a smaller area of the lens constructs the entire image.
Similarly, for a person that has had the natural lens of the eye removed because of a cataract condition and an intraocular lens inserted as a replacement, the ability to adjust the lens (accommodate) to the distance of the object being viewed is totally absent. In this case, the lens provided is usually set at the singe infinity distance focal power and spectacles are worn to provide the additional positive optical power needed for in-focus close vision. For such a patient, a functional multifocal lens would be particularly useful.
It is known in the art that under certain circumstances that the brain can discriminate separate competing images by accepting the in-focus image and rejecting the out-of-focus image.
One example of this type of lens used for the correction of presbyopia by providing simultaneous near and far vision is described in U.S. Pat. No. 4,923,296 to Erickson. Described therein is a lens system which comprises a pair of contact lenses each having equal areas of near and distant optical power, the lens for one eye with a near upper half and a distant lower half while the lens for the other eye contains a distant upper half and near lower half. Together these are said to provide at least partial clear images in both eyes, and through suppression by the brain of the blurred images, allows alignment of the clear image to produce an in-focus image.
U.S. Pat. No. 4,890,913 to de Carle describes a bifocal contact lens comprising a number of annular zones having different optical powers. The object in the design of this lens is to maintain, at all times regardless of pupil diameter, an approximately equal division between near and distant powers, requiring on the lens between 6 and 12 total zones.
Another attempt at providing a bifocal contact lens is described in U.S. Pat. No. 4,704,016 to de Carle. Again, this lens attempts to maintain, at all times regardless of pupil diameter, an approximately equal division between near and distant powers.
Another approach to producing a multifocal corrective eye lens involves the use of diffractive optics. One of the shortcomings of this approach has been a deficiency in vision at low light levels. In a diffractive design only about 40% of the light incident on the lens is used for near vision with another 40% being used for far vision. The remaining 20% is not used for either near or far vision, but rather is lost to higher orders of diffraction and scatter effect. This represents the best theoretical case and in manufacturing reality even less light is available due to manufacturing difficulties. Difficulty of manufacture in general represents another shortcoming of diffractive lenses since the diffractive surface must be to tolerances on the order of the wavelength of light.
One attempt known in the art to provide a method of compensating for presbyopia without complex lens manufacture is known as "monovision". In the monovision system a patient is fitted with one contact lens for distant vision in one eye and a second contact lens for near vision in the other eye. Although it has been found that with monovision a patient can acceptably distinguish both distance and near objects, there is a substantial loss of binocularity, i.e. depth perception.
Although simple systems such as monovision are somewhat understood, more complex schemes for multifocal refractive lenses are primarily theoretical.
U.S. Pat. Nos. 5,002,382 and 5,024,517 both issued to Seidner, disclose complementary pairs of contact lenses having two or more corrective optical powers in opposite configurations. Both of the lens pairs described are closely related to the monovision concept: the pair has a central power disparity between lenses, and fails to effect a power ratio adjustment as a function of illumination.
European Patent Application, Publication number 0 201 231 A2 by Ho, et al. describes a binocular pair of contact lenses having a plurality of optical zones that are complementary between lenses, i.e. opposite alternate zones of near and distance powers.
A more practical and improved approach to providing a multi-focal ophthalmic lens is described in copending application Ser. No. 7/827,199 filed on Jan. 28, 1992. In this application there is disclosed a multifocal ophthalmic lens characterized by having a central zone wherein one of the multifocal segments includes the central zone of the lens. The boundary between the segments is defined by an arcuate path such as a semi-circle having both ends of the path on the adjoining parameter of the near and distant segments to eliminate from the central optical axis the segment boundaries including the central junction point.
While the lenses made according to the above described applications are functional and the manufacturing techniques described therein are a practical way of molding ophthalmic lenses, an important aspect of proper multifocal vision under various light conditions has not been met.
Under real-world conditions, the ability of the brain to discriminate between (or among) two or more images where only one is in-focus is greatly enhanced if the central part of the image consists of only one focal length. It has also been found that high levels of illumination typically are accompanied by distance vision situations.
It is an object, therefore, of the present invention to provide a pair of ophthalmic lenses for a presbyope that yields improved visual acuity in general, and particularly under high intensity light conditions.
It is a further object of the invention to describe a method for determining the manner in which such lenses are to be fitted to a patient to produce the desired improvement in vision, especially by matching the optical power required for a high illumination situation.